Cascade cycle for liquefaction of natural gas



United States Patent 3,315,477 CASCADE CYCLE FOR LIQUEFACTION OF NATURALGAS Jackson 0. Carr, Shawnee Mission, Kans., assignor to ConchInternational Methane Limited, Nassau, Bahamas, a company of the BahamasFiled July 15, 1964, Ser. No. 382,774 4 Claims. (Cl. 62-23) Thisinvention relates to the liquefaction of gas and more particularly to amethod for the liquefaction of natural gas, which is normally composedmostly of methane but may contain relatively small proportions of bothhigher hydrocarbons and lower boiling-point components such as nitrogen.The general system is similar, but represents an improvement over thatdisclosed in US. Patent No. 3,020,723 to De Lury et al. The invention isparticularly useful for liquefying natural gas that is lean incomponents which exhibit higher vapor pressures than methane (such asnitrogen, hydrogen and helium).

According to the invention, natural gas enters the plant at a highpressure and is cooled and liquefied by heat exchange in closed systemswith an ethylene, propane and methane cascade refrigeration system, asis also done in the De Lury patent mentioned above. However, instead ofemploying a closed system of pure methane refrigerant in the finalrefrigeration stages, the final cooling is achieved by flashing theliquefied natural gas, and the flash vapors are then re-cornpressed andrecycled to the inlet feed gas stream, thereby eliminating the separateheat exchangers previously needed to cool and condense the methanerefrigerant. In addition, a portion of the liquefied gas is withdrawnfrom the main stream prior to subcooling for tempering the cold flashgases which are subsequently compressed for recycling to the inlet gasfeed stream mentioned above to avoid feeding extremely cold gas to thecompressors. The use of flash drums in the low levels eliminates morecostly heat exchangers.

As is apparent from the above, the main object is to reduce the powerrequirement for liquefying natural gas, to simplify the process andreduce the equipment required.

The specific nature of my invention, as Well as other objects andadvantages thereof, will clearly appear from a description of apreferred embodiment as shown in the accompanying drawing in which thefigure is a flow diagram illustrating the process of the invention forthe liquefaction of natural gas.

While the flow chart of the drawing shows in detail one system for thepractice of the invention, with temperatures and pressures given for atypical type of gas at a particular initial pressure and temperature, itwill be understood that this is merely exemplary, and that the operatingconditions and subdivision of stages may be varied in accordance withknown design considerations to suit particular conditions and materials.In the example, the gas to he liquefied is assumed to be a lean naturalgas relatively free of components that exhibit higher vapor pressurethan methane, which is the principal ingredient, and which is assumed toenter the flow line 1 at a temperature of 70 F. and at a pressure of 600p.s.i.a. The refrigeration cycle for reducing the compressed gas to aliquefied state is a modified cascade system having a sequence ofrefrigeration steps including a propane refrigeration cycle, an ethylenerefrigeration cycle, and a methane refrigeration cycle, as indicated inthe drawing. Since the propane and ethylene cycles are generally similarto those employed in the De Lury Patent No. 3,020,- 723 mentioned above,they will be described only briefly. The temperatures and thesignificant pressures at various stages are as indicated in the drawing.

Gas entering at 1 is initially cooled to a temperature of approximately30 F. in heat exchanger 2 and to a temperature of approximately -31 F.in heat exchanger 3, by heat exchange with evaporating propane from thepropane cycle. The propane is stored in container 4 at F. and at apressure of p.s.i.a. This is taken in line 6 through throttle valve 7 tosupply propane refrigerant to heat exchanger 2 and the pressure isreduced to produce the indicated temperature in the feed gas line .1,the propane being reduced in temperature to 25 F., at which temperaturethe propane vapor is conducted fromthe shell of heat exchanger 2 in line8 into line 9, and hence to second stage compressor 11, where itspressure is raised from 60.6 p.s.i.a. to 140 p.s.i.a. and condensed at70 F. by 'heat exchange with cooling water in exchanger 12 and returnedto the storage container 4. Some of the propane is taken from the shellof heat exchanger 2 in line 14, further expanded through throttle valve16 for the second heat exchange step in exchanger 3, and similarlyreturned to first stage compressor 17, where its pressure is raised from16.4 p.s.i.a. to 60.6 p.s.i.a., at which pressure it joins the propanefrom line 9 for second stage compression at 11 as previously indicated.Some of the propane from line 6 is also passed through throttle valve 18to reduce its pressure for use in heat exchanger 19 for cooling ethylenefrom the last compression stage of the ethylene cycle, from atemperature of 70 F., and then joins the stream from line 9 in line 9 aspreviously described. Propane from line 14 is similarly reduced inpressure by throttle valve 21 and evaporated in heat exchanger 22 priorto rejoining the stream from heat exchanger 3, for a second stage ofcooling which reduces the temperature of the ethylene in line 23 to atemperature of 23 F., at which temperature it is condensed and is storedin ethylene container 24. The natural gas in line 1 now continuesthrough several similar stages of ethylene heat exchange in exchanges25, 26, 27 and 28, with further temperature reductions at each stage asindicated lay the drawing. The ethylene in turn is similarly compressedin a series of compressors (which may be a multi-stage compressor) asindicated in the flow chart, the number of stages and the temperaturesfor each stage being selected in accordance with known principles. Itwill be understood that the various throttle valves, e.g., valves 13,17, 21, 16, etc., are part of conventional level control devices used tomaintain the proper liquid level in their respective heat exchangers.

After passing through heat exchanger 28, the feed gas is at atemperature of 144 F., and being still at a high pressure, it is nowbelow its critical temperature and in liquid phase. The main stream ofnow liquid natural gas continues through three more heat exchange stagesin heat exchangers 31, 32 and 33 respectively, to reduce its temperatureto approximately 195 F., at which point it is passed through throttlevalve 35 into flash drum 36. However, a portion of the stream isdiverted between heat exchangers 28 and 31 in line 37 for heat exchangewith the cold flash vapors from flash drums 36 and 38, to recover therefrigeration in these vapors, and to temper the gases going into thecompressors, as will now be described.

After passing through throttle valve 35, the liquefied feed gas isflashed in flash drum 36 on reducing the pressure to 70 p.s.i.a. thuslowering its temperature to 216 F. It is then passed through throttlevalve 39 into second flash drum 38, Where its pressure is reduced to 27p.s.i.a., thus lowering the temperature to 242 F.

Liquefied feed gas from line 37 is heat exchanged in exchanger 41 withthe flash vapors from flash drum 36 to raise the temperature of theflash vapor in line 42 to F., and correspondingly cool the liquidmethane, which is then passed through throttle valve 43 to control therate of flow returned to the main stream in line 1 and thence into flashdrum 36, together with the main stream of liquefied feed gas in line 1.Similarly, liquefied feed gas in line 37 is withdrawn in line 44 forheat exchange in exchanger 46 with flash vapors from flash drum 38, andthen passed through throttle valve 47 into flash drum 38. The remainingliquefied feed gas in line 37 is used in heat exchanger 48 to temper thevapors in line 49, which are now substantially at atmospheric pressure.

Liquefied gas at 242 F. from flash drum 38 continues on main line 1through final stage heat exchanger 51, where its temperature is reducedto 251 F. by heat exchange with evaporating liquid withdrawn from line 1through throttle valve 52. A final pressure reduction stage and throttlevalve 53 reduces the pressure of the liquefied gas to slightly aboveatmospheric pressure, and a temperature of 25 8 F.-, for storage in asuitable storage facility 54, which may be an iu-ground large-scalestorage tank, or any other suitable storage facility, in which it isstored essentially as a boiling liquid at slightly above atmosphericpressure. Since it is in a boiling condition, vapors are removed on line56 from the storage tank 54 and compressed by a low pressure blower 57into line 58, where it joins the vapor stream in line 49 for the firststage of compression by methane compressors 59, 60, 61, which may be amultistage single compressor, for raising the pressure of the methane inlines 49, 45 and 42 respectively as indicated in the flow chart. Afterleaving compressor 61, the methane, now at 275 p.s.i.a. and aboveambient temperature is cooled by cooling water heat exchange inexchanger 63 to 70 R, where some of it may be used on line 64 as plantfuel. The remainder of the methane is passed in line 66 throughadditional compressor stage 67 to line 68, and after further watercooling in exchanger 69, is returned on line 71 at a temperature of 70F. and at the pressure of the incoming natural gas, to join the feed gasstream on line 1. It will be noted that any small amount of impuritiesof higher vapor pressure than methane (e.g., nitrogen) will ultimatelybe removed on line 64 with the feed gas stream, and therefore do notrequire special treatment. For high concentrations of high .vaporpressure impurities, the impurities may be removed by conventionalmeans.

It will be apparent that the embodiments shown are only exemplary andthat various modifications can be made in construction and arrangementwithin the scope of my invention as defined in the appended claims.

I claim:

1. The method of liquefying a gas comprising (a) supplying the gas in amain feed stream at high pressure and essentially ambient temperature,

(b) removing heat from the gas to cool the gas to a temperature at whichit is in a liquid state at said high pressure, 7

(c) withdrawing from said main feed stream a portion of the liquefiedgas from step (b) as a side stream,

(d) subcooling the rest of the gas in said main stream to a still lowertempearture than the gas in the side stream,

(e) throttling said subcooled liquid gas from step (d) into a flash drumat a reduced pressure to flash off part of the gas as a vapor and stillfurther cool the residual liquid gas,

(f) heat exchanging liquefied gas from said side stream of step (c) withthe flash vapor from step (e) to further subcool said liquefied sidestream gas,

(g) throttling said liquefied subcooled side stream gas of step (t) intothe flash drum to rejoin the main stream, 7

(h) throttling the subcooled main stream liquefied gas into a storagecontainer as liquefied gas at substantially atmospheric pressure,

(i) withdrawing vapor from said storage container to maintain thepressure in said container at substantially atmospheric pressure,

(j) compressing said vapor in multi-stage compression and cooling saidcompressed vapor to ambient temperature,

(k) withdrawing part of said compressed vapor for use as product, and

(1) further compressing the remainder of said compressed vapor to aboutthe initial pressure of the main feed stream, and returning it to theentry point (c) withdrawing from said main feed stream a portion of theliquefied gas from step (b) as a side stream,

(d) subcooling the rest of the gas in said main stream to a still lowertemperature than the gas in the side stream,

(e) throttling said subcooled liquid gas from step (d) into a flash drumat a reduced pressure to flash 011? part of the gas as a vapor and stillfurther cool the residual liquid gas,

(f) heat exchanging liquefied gas from said side stream of step (c) withthe flash vapor from step (e) to further subcool said liquefied sidestream gas,

(g) throttling said liquefied subcooled side stream gas of step (t) intothe flash drum to rejoin the main 7 V I stream,

(h) throttling the subcooled main stream liquefied gas' into a storagecontainer as liquefied gas at substantially atmospheric pressure,

(i) removing a small portion of the main fuel stream after said flashdrum subcooling, (j) reducing the pressure of said removed small portionand passing it at substantially atmospheric pressure in heat-exchangerelationship with the main stream to further subcool the main streamprior to storage, while expanding said small portion to a vaporcondition,

(k) heat exchanging liquefied gas from said side stream with the vaporfrom the preceding step to further cool said side stream, and

(l) returning the further cooled side stream from the preceding step tothe main stream.

4. The method of liquefying a gas comprising (a) supplying the gas in amain feed stream at high pressure and essentially ambient temperature,

(b) removing heat from the gas to cool the gas to a temperature at whichit is in a liquid state at 'said high pressure,

(c) withdrawing'from said main feed stream a portion of the liquefiedgas from step (b) as a side stream,

(d) subcooling the rest of the gas in said main stream to a still lowertemperature than the gas in the side stream,

(e) throttling said subcooled liquid gas from step (d) into a flash drumat a reduced pressure to flash ofi part of the gas as a vapor and stillfurther cool the residual liquid gas,

(f) heat exchanging liquefied gas from said stream of step (c) with theflash vapor from step (e) to further subcool said liquefied side streamgas,

(g) throttling said liquefied subcooled side stream gas of step (f) intothe flash drum to rejoin the main stream,

(h) throttling the subcooled main stream liquefied gas into a storagecontainer as liquefied gas at substantially atmospheric pressure,

(i) further throttling the residual liquid gas from step (e) into asecond flash drum at a further reduced pressure to produce vapor at alower pressure than in step (e) and to further subcool the residualliquid (j) heat exchanging liquefied gas from the side stream of step(c) with the flash vapor from the preceding step, and

(k) throttling said liquefied subcooled side stream from the mainstream.

References Cited by the Examiner UNITED STATES PATENTS NORMAN YUDKOFF,Primary Examiner. the preceding step into the second flash drum torejoin 10 V. W. PRETKA, Assistant Examin'er.

1. THE METHOD OF LIQUEFYING A GAS COMPRISING (A) SUPPLYING THE GAS IN AMAIN FEED STREAM AT HIGH PRESSURE AND ESSENTIALLY AMBIENT TEMPERATURE,(B) REMOVING HEAT FROM THE GAS TO COOL THE GAS TO A TEMPERTURE AT WHICHIT IS IN A LIQUID STATE AT SAID HIGH PRESSURE, (C) WITHDRAWING FROM SAIDMAIN FEED STREAM A PORTION OF THE LIQUEFIED GAS FROM STEP (B) AND A SIDESTREAM, (D) SUBCOOLING THE REST OF THE GAS IN SAID MAIN STREAM TO ASTILL LOWER TEMPERATURE THAN THE GAS IN THE SIDE STREAM, (E) THROTTLINGSAID SUBCOOLING LIQUID GAS FROM STEP (D) INTO A FLASH DRUM AT A REDUCEDPRESSURE TO FLASH OFF PART OF THE GAS AS A VAPOR AND STILL FURTHER COOLTHE RESIDUAL LIQUID GAS, (F) HEAT EXCHANGING LIQUEFIED GAS FROM SAIDSIDE STREAM OF STEP (C) WITH THE FLASH VAPOR FRM STEP (E) TO FURTHERSUBCOOL SAID LIQUEFIED SIDE STEAM GAS, (G) THROTTLING SAID LIQUEFIEDSUBCOOLED SIDE STREAM GAS OF STEP (F) INTO THE FLASH DRUM TO REJOIN THEMAIN STREAM, (H) THROTTLING THE SUBCOOLED MAIN STREAM LIQUEFIED GAS INTOA STORAGE CONTAINER AS LIQUEFIED GAS AT SUBSTANTIALLY ATMOSPHERICPRESSURE, (I) WITHDRAWING VAPOR FROM SAID STORAGE CONTAINER TO MAINTAINTHE PRESSURE IN SAID CONTAINER AT SUBSTANTIALLY ATMOSPHERIC PRESSURE,(J) COMPRESSING SAID VAPOR IN MULTI-STAGE COMPRESSION AND COOLING SAIDCOMPRESSED VAPOR TO AMBIENT TEMPETATURE, (K) WITHDRAWING PART OF SAIDCOMPRESSED VAPOR FOR USE AS PRODUCT, AND (L) FURTHER COMPRESSING THEREMAINDER OF SAID COMPRESSED VAPOR TO ABOUT THE INITIAL PRESSURE OF THEMAIN FEED STREAM, AND RETURNING IT TO THE ENTRY POINT OF THE MAIN FEEDSTREAM.